1. Technical Field
The present invention relates to an apparatus for measuring residual stress of an optical fiber. More particularly, the present invention relates to an apparatus for measuring a residual stress of an optical fiber which is provided with a variable polarization device of which rotation is unnecessary to measure the residual stress in high resolution and at high speed instead of a rotary polarizer or analyzer.
2. Related Art
Optical communication is a communication system that transmits and receives light and signals through an optical fiber that is composed of a core having a high refractive index and a cladding having a low refractive index.
Corning Inc. developed an optical fiber having a transmission loss of 20 dB/km in 1970, which resulted in practically utilizing the optical communication. In the optical communication, a transmission terminal converts an electrical signal into a light signal and transmits the light signal through the optical fiber and a reception terminal reconverts the light signal into the electrical signal. The optical communication has an advantage in that there is no interference by external electromagnetic waves, wiretapping is difficult, and a large amount of information can be processed at the same time in comparison with electrical communication. The application area of the optical communication shows a tendency for growth.
The optical fiber is generally manufactured by a high-temperature drawing process. At this time, stress is generated. The stress is not completely removed and a part thereof resides after the optical fiber is manufactured. This is called residual stress. Accordingly, a technique to accurately measure the distribution and magnitude of the residual stress in the optical fiber and adjust the residual stress to an optimum state is required.
Meanwhile, a photoelastic effect is changed depending on the direction of stress which resides in a medium and a refractive index of the optical fiber or an optical fiber preform is changed depending on a polarization of light by the photoelastic effect.
The residual stress increases optical loss due to light scattering of the optical fiber and causes the refractive index to be changed by the photoelastic effect. The change of the refractive index of the optical fiber is a primary element for determining waveguide characteristics of an optical signal in the optical fiber. It is possible to accurately grasp optical characteristics of the optical fiber by the change of the refractive index. Accordingly, it is very important to measure the residual stress in order to produce high-quality optical fibers, develop special optical fibers, and research characteristics thereof.
FIG. 1 is a conceptual diagram of a known system for measuring residual stress of an optical fiber.
Light input from a light source passes through an optical fiber 30 through a rotary polarizer 10 and a wave plate 20. The light passing through the optical fiber 30 has a phase difference depending on distribution of stress in the optical fiber 30. The light passes through an analyzer 40, whereby the phase difference is displayed by optical intensity. The light passing through the analyzer 40 is input into a CCD camera to be converted to the electrical signal. The residual stress of the optical fiber is measured through such a process. Although not shown in the figure, the same principle is applied even to a system in which the analyzer rotates instead of the polarizer.
In the case of measuring the residual stress of the optical fiber as described above, a position where the intensity of the CCD camera is 0 is searched while rotating the polarizer or the analyzer using a motor and a rotational angle of the polarizer or the analyzer must be acquired when the intensity is 0. Therefore, it takes a long time to measure the residual stress of the optical fiber.